Their answers are exactly the same — one of them from the other.
A. must copy
B. must have copied
C. should copy
D. should have copied
网友回答
A. must copy
(答案→)B. must have copied
C. should copy
D. should have copied
解析：考查情态动词。must have done 表示对过去事情肯定的推测，句意：他们的答案确切的说是一样的。------他们当中的一个人一定抄了另一个人的。所以选B。